Logic, it is said, is normative - logic tells us something about how we ought to think or reason. However, it's not clear why we should think logically, or which rules or which theory we are meant to follow. In the following chapters, I provide a novel view of logical normativity which aims to answer these vexing questions. I defend the view that logical normativity can be understood on the basis of a conception of logic according to which its laws are fundamentally descriptive, what I call the 'laws of truth' conception of logic. But I also defend the view that logical normativity can be understood on the basis of a conception of logic according to which its laws are fundamentally prescriptive, what I call the 'laws of thought' conception of logic. The resulting view is what I call 'logical normativity dualism', the idea that logic is normative in both of these distinct yet complementary ways.
In Chapter 1, I address some of the fundamental issues of logical normativity by uncovering the deep distinction between the laws of truth and laws of thought conceptions of logic. I employ the figure of the logical alien as it appears in the work of Gottlob Frege in order to draw out these alternative conceptions, showing that ultimately Frege's view presumes the laws of truth conception, but differently from Frege, Kant and the later Wittgenstein's conception of logic characterizes the logical laws as explicit prescriptions for how we ought to reason. I conclude that the contemporary literature on logical normativity has been shaped primarily by Frege's laws of truth conception of logic, but that we should return to a Kantian view in order to get a complete picture of logical normativity.
In Chapter 2, I present 'constitutive normativism', an explication of the normativity of logic based in the laws of thought conception of logic, and I defend it against a number of pressing objections. Constitutive normativism posits the evaluative, wide scope 'in spirit' normative requirement on deductive propositional reasoning that it should avoid certain combinations of attitudes which violate modus ponens and universal instantiation. I also argue that Gilbert Harman's well-known objections to logical normativity about belief revision, excessive cognitive demands, and triviality, can be handled by constitutive normativism. I conclude by defending constitutive normativism against some objections to the general strategy of constitutivism as a way to explain the categorical authority of norms.
In the second half of my dissertation, I develop and employ the logical normativity dualism framework to address some standing problems in the philosophy of logic. One key upshot which I discuss in Chapter 3 is that constitutive normativism can help solve a recent problem for Quinean, anti-exceptionalist views of logic, what is called the Adoption Problem. I explain that the Adoption Problem poses a problem for the Quinean view because we aren't able to reason in accordance with our preferred theory without using modus ponens and universal instantiation, regardless of whether either inference rule is valid in our preferred theory. But by embracing a plurality of conceptions of logic - taking on both the laws of truth and laws of thought conceptions of logic - we can distinguish between kinds of logical rules which make inquiry possible and those which we inquire into, and thereby save the Quinean view.
In Chapter 4, I complete the picture of logical normativity dualism by developing and defending an evidentialist explication of logical normativity on the basis of the laws of truth conception of logic. I demonstrate that the two logical normativities, constitutive normativism and what I call 'evidential reasons normativism', complement each other rather than conflict. I also draw on the developments of the previous chapters to demonstrate that the supposed incompatibility between logical pluralism and logical normativity doesn't carry over to my view.
